For example, in a liquid crystal display (“LCD”) device the light signal of each individual display element does not emanate as a point source located on the front surface of the device. Instead, the light signal originates in one or more backlight sources positioned behind the front surface. The light signal passes through a veritable tunnel of layers, including a layer of angularly-twistable liquid crystals, so the light apparently emanates from a source internal to the device. The amount of angular twisting of the liquid crystal fluid material changes as a function of a voltage applied to an individual display element. This voltage is usually referred to as the “gray scale voltage.” More often, in fact, the voltage is normalized so that the full range of gray scale levels is represented by a number of subdivisions equal to a power of two. Most typically, gray scale levels are expressed as a number between 0 to 63 or between 0 to 255. At a gray scale level of “0”, the display element shows its darkest black; at the highest level of “63” or “255”, the display element shows its brightest white. As a result of these and other effects, a user's ability to perceive both the brightness and the color of the overall screen is a function of the user's position relative to a line projecting normal from the center of the front surface.
Because of the various effects described above, any viewer of an active matrix LCD who is not located precisely on a normal projecting outwardly from the center of the screen will have a view of the screen that is impaired to some degree. Because it is difficult for an individual user to maintain this ideal position precisely, and because it is impossible for more than one user of the screen to simultaneously occupy this ideal position, there will almost always be consequences from the non-ideal LCD screens in common use.
One of these consequences is an effect generally referred to as “gray scale inversion.” When the gray scale voltage applied to the individual display elements increases, the luminance of the screen perceived by a viewer at any particular point should increase monotonically, that is, the luminance should consistently increase without oscillations in which decreases occur. This is not to say that the rate of increase will be constant. In fact, as taught by one of the present inventors in U.S. Pat. No. 6,809,746 at FIG. 8, the transmission or luminance of a screen usually increases in an “S”-shape manner as gray scale increases.
There are, however, angular positions in front of an LCD screen where the measured luminance will decrease as the gray scale value is increased. Such positions are said to experience “gray scale inversion” at that particular gray scale level. Most typically, these gray scale inversions will occur, if at all, towards the lower end of the gray scale continuum.
A related property of a screen of this type is “viewing angle,” which is an often mis-understood concept. Many people believe viewing angle to be the maximum angle at which one can view a screen without losing brightness or color shifts. Actually, viewing angle is related to “contrast ratio,” which is, in turn, defined as the ratio of the brightness of a screen when all display elements are set to a maximum gray scale level (the screen is “white”) to the brightness of the screen when all display elements are set to a minimum gray scale level (the screen in “black”). A “straight on” viewer may have a contrast ratio of at least 250:1. As the viewer moves off center, the contrast ratio decreases. One is considered to be within the “viewing angle” as long as the contrast ratio exceeds an arbitrary value, typically 10:1 or 5:1. Assuming certain symmetries of the screen, if a viewer can move 70° off center before the contrast ratio declines from 250:1 to 10:1, then the screen is said to have a viewing angle of 140°, since a symmetrical screen would be in the desired contrast ratio range from a viewer inclination between 70° to one side to 70° to the other side. It is often observed that the “reading angle” for a screen is typically smaller than the viewing angle, since the minimum acceptable contrast ratio for reading information from a screen is larger than 10:1.
Gray scale inversion is always undesirable, but in many applications, it can be absolutely unacceptable, such as when the screen is used as an avionics display in an aircraft operating under instrument flight rules. In such cases, it is critical to be able to rapidly and reliably determine the performance characteristics of a display under test (DUT) so that its limitations are resolved (or at least known) prior to implementation.
In addition to the novel features and advantages mentioned above, other features and advantages will be readily apparent from the following descriptions of the drawings and exemplary embodiments.